Lens system for projection television receivers

ABSTRACT

An optical system of a projection television receiver for projecting pictures from a picture tube onto a screen, comprising a fluorescent face constituting the tube face of the receiver tube having a nonspherical shape in such a direction as to compensate the curve of the image face of the projected picture on the screen, and further comprising lenses arranged from the screen side to the tube face of the picture tube in the order of: 
     (A) a first lens having relatively weak power (reciprocal of focal distance) and having a nonspherical shape at least at one face; 
     (B) a second lens having relatively strong positive power; 
     (C) a third lens having relatively weak power and having a nonspherical shape at least at one face; and 
     (D) a fourth lens having relatively strong negative power and having a concave shape at the face of the screen side.

BACKGROUND OF THE INVENTION

The present invention relates to a projection television optical systemfor projecting pictures from a picture tube in a projection televisionreceiver onto a screen.

A demand for increase in the size of the picture reproduction face of atelevision receiver aiming at powerful pictures which cannot be obtainedon a small-sized picture plane has been intensified. Thus so-calledprojection television receivers have been widely brought into use. Inthe projection television receiver, pictures reproduced on thefluorescent face of a Braun tube (picture tube) are enlarged andprojected onto a screen by a projection optical system including lensesand mirrors, pictures being obtained in a large picture plane.

As a result of various improvements heretofore applied to suchprojection television receivers, a great advance is found as comparedwith several years ago. Lenses play a very important role in thisprojection television receiver.

Lenses according to a proposal disclosed in U.S. Pat. No. 4,348,081 or4,300,817 provide high luminance which is 1.0 in F number although onlythree sheets of lens are used. This is achieved by the design utilizingat its maximum the fact that the plastic lens can be easily formed to anonspherical shape.

It is no exaggeration to say that the projection television receiver isgreatly developed owing to the lens disclosed in the aforementioned U.S.Patents. Under the existing circumstances, however, the projectiontelevision receivers are still inferior to the direct-view televisionreceivers in picture quality, compactness and price.

Respective problems will now be described.

(1) Picture quality: Important picture quality terms of the projectiontelevision receiver are focus, brightness and contrast. The focus of alens can be generally estimated by using the MTF (modulation transferfunction). As a result of estimating the lens described in theaforementioned U.S. Pat. No. 4,348,081, the focus was very fine at thecentral portion of the picture plane. In the actual Braun tube, however,the luminous spectrum characteristics of the fluorescent substance havea discrete distribution. By taking this into consideration, the MTF isworsened. Especially in the low frequency region, the MTF of the lens isinferior to that of the glass lens generally used.

This is caused by the fact that regard to improvement of the coloraberration is not paid in the lens according to the proposal describedin the aforementioned U.S. Patent. If the color aberration was improved,the MTF would be largely improved.

Since all of the lenses according to the aforementioned proposal arecomposed of plastic lens, a change in temperature causes a change inrefractive index, resulting in focus deterioration.

In order to increase the input power of the projection tube and improveits brightness, so-called liquid-cooled Braun tubes are recently used.In these liquid-cooled Braun tubes, the heat generated by increase ininput power is cooled down. The heat is radiated from a radiation fin bythe convection of the liquid to reduce and make uniform the temperatureof the glass of the fluorescent face, the glass of the fluorescent facebeing prevented from breaking due to high temperature.

(2) Compactness: In order to achieve compactness, the number of sheetsof mirror in the set has heretofore been increased. Under the existingcircumstances, however, the number of sheets in use has already reachedits limit. For achieving further compactness, therefore, the projectiondistance and the lens length must be shortened. At the same time, themirror arrangement must also be reconsidered.

In this connection, the projection distance and the lens length of thelens according to the proposal described in U.S. Pat. No. 4,348,081 arerespectively 1,200 to 1,300 mm and approximately 200 mm when a screen of45 inches is used and the magnification is 9 to 10.

(3) Price: As described above, the conventional projection televisionset has problems such as a complicated mirror arrangement, large lensdimensions, use of a liquid-cooled Braun tube, and a complicatedcoupling structure between the Braun tube and the concave lens. As aresult, the high cost is inevitable.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical system forprojection television receiver capable of shortening the projectiondistance and the lens length, improving the picture quality such asfocus, brightness and contrast, realizing a compact projectiontelevision set, and reducing the cost.

In order to achieve the above described object, an optical systemaccording to the present invention is composed of lenses having widepicture angles. That is to say, a nonspherical lens having weak power, alens having convex faces at both sides and occupying the major portionof the entire power, a nonspherical convex lens having weak power, aconcave lens having large negative power, and a fluorescent face of theBraun tube having a nonspherical shape are sequentially arranged fromthe screen side.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating an embodiment of the presentinvention.

FIG. 2 is a diagram for illustrating the definition of the face shape.

FIGS. 3 to 6 are characteristic diagrams illustrating MTFcharacteristics of embodiments according to the present invention.

FIG. 7 is a distortion characteristic diagram of an embodiment accordingto the present invention.

FIGS. 8(a) and (b) and 9 (a) and (B) are raster distortioncharacteristic diagrams of embodiments according to the presentinvention.

FIG. 10 is a sectional view of another embodiment according to thepresent invention.

FIGS. 11 to 14 are characteristic diagrams illustrating MTFcharacteristic of the embodiment shown in FIG. 10.

FIG. 15 is a distortion characteristic diagram of an embodimentaccording to the present invention.

FIG. 16 is a diagram for illustrating the nonspherical shape of thefluorescent face.

FIG. 17 is a luminous spectral characteristic diagram of a fluorescentsubstance.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail by referring toembodiments.

In accordance with the present invention, the focal distance is largelyreduced as compared with the prior art in order to solve the abovedescribed problems. Although the magnification is approximately the sameas that of the prior art, the picture angle is largely widened to 45°from approximately 25° of the prior art.

In general, such a large picture angle makes the lense design verydifficult. In particular, it becomes difficult to make the brightnessand focus of the periphery portion have desired performance values. If,on the contrary, the picture angle can be increased and the projectiondistance can be shortened, the focal distance can also be shortened. Inthis case, the reduced color aberration results in an improved focus anda small-sized set. And the small-sized lens results in a reduced cost.

In case of the lens of the type disclosed in U.S. Pat. No. 4,348,081,much consideration is not paid to the color aberration on the centralaxis. In general, the color aberration on the axis is improved bycombining a concave lens and a convex lens having which are differenteach other in dispersion. In case of a projection television receiver,however, color aberration does not pose a hard problem as compared withother optical devices such as a still camera, because red, blue andgreen monochromatic Braun tubes are used.

In reality, however, the luminous distribution of the fluorescentsubstance is not a bright line completely. (See FIG. 17.) In the typedescribed in U.S. Pat. No. 4,348,081, the increment Δd in spot diametercaused by color aberration at the center of the screen is represented asΔd=(1+M)².f/F.M.ΔN/N-1 ##EQU1## where M, F, f and N are magnification, Fnumber, focal distance and refractive index, respectively.

It is seen from the expression that Δd is reduced in proportion to thefocal distance.

FIG. 1 is a sectional view illustrating an embodiment of an opticalsystem for projection television receiver according to the presentinvention. A first lens 13, a second lens 14, a third lens 15, a fourthlens 16, a glass bulb 17, a fluorescent face 18, an optical axis 19, aBraun tube 20, and liquid 8 are shown in FIG. 1.

The embodiment illustrated in FIG. 1 includes the first lens 13 composedof a nonspherical lens having weak power (the reciprocal of the focaldistance), the second lens 14 composed of a lens having convex surfacesat both sides and occupying the major portion of the power of the wholeoptical system, the third lens 15 composed of a nonspherical convex lenshaving weak power, the fourth lens 16 composed of a concave lens havinglarge negative power, and the fluorescent face 18 of the Braun tubehaving a spherical or nonspherical shape are arranged in this order fromthe side of the screen which is not illustrated. While the glass bulb 17has a flat face at the side of liquid 8, the fluorescent face 18opposing thereto has a spherical or nonspehrical shape.

The lens data of respective lenses arranged as shown in FIG. 1 will nowbe described.

Table 1 shows concrete examples of those lens data.

                                      TABLE 1                                     __________________________________________________________________________                  Radius of   Space                                               Paraxial      Curvature                                                                            Clap Between                                                                             Refractive                                    system        (RD)   Radius                                                                             Faces Index                                         __________________________________________________________________________    Screen        ∞                                                                              600                                                                                701.0 1.0                                           First Lens S.sub.1                                                                          92.9391                                                                              37.0                                                                               7.0487                                                                              1.4936                                                   S.sub.2                                                                          123.690                                                                              35.5                                                                               16.5904                                                                             1.0                                           Second Lens                                                                              S.sub.3                                                                          86.300 34.0                                                                               17.3  1.62286                                                  S.sub.4                                                                          -101.822                                                                             32.0                                                                               29.6586                                                                             1.0                                           Third Lens S.sub.5                                                                          181.3277                                                                             34.0                                                                               8.012 1.4936                                                   S.sub.6                                                                          -152.796                                                                             34.0                                                                               17.6  1.0                                           Fourth Lens                                                                              S.sub.7                                                                          -40.234                                                                              34.5                                                                S.sub.8        3.0   1.5401                                                      ∞                                                                              60.0                                                     Cooling Liquid            4.2   1.41                                                        ∞                                                                              60.0                                                     Glass                     13.8  1.5401                                        Fluorescent Face S.sub.10                                                                   -119.63                                                                              60.0                                                     __________________________________________________________________________    Aspherical                                                                    coeffi-                                                                       cients     CC    AD   AE    AF   AG                                           __________________________________________________________________________    First Lens       × 10.sup.-6                                                                  × 10.sup.-9                                                                   × 10.sup.-12                                                                 × 10.sup. -16                                   S.sub.1                                                                         -6.3741                                                                             -1.7690                                                                            -3.4450                                                                             2.5723                                                                             -5.3853                                                       × 10.sup.-6                                                                  × 10.sup.-9                                                                   × 10.sup.-12                                                                 × 10.sup.-16                                    S.sub.2                                                                         0.2738                                                                              -1.8085                                                                            -2.5767                                                                             2.6339                                                                             -5.7156                                      Third Lens       × 10.sup.-6                                                                  × 10.sup.-10                                                                  × 10.sup.-13                                                                 × 10.sup.-17                                    S.sub.5                                                                         -138.085                                                                            1.8057                                                                             -7.4569                                                                             -5.7472                                                                            7.5759                                                        ×10.sup.-8                                                                   × 10.sup.-9                                                                   × 10.sup.-12                                                                 × 10.sup.-6                                     S.sub.6                                                                         -1.8908                                                                             4.8071                                                                             1.2516                                                                              -1.4590                                                                            1.5522                                       Fluorescent Face × 10.sup.-7                                                                  × 10.sup.-11                                                                  × 10.sup.-15                                                                 × 10.sup.-16                                    S.sub.10                                                                        -3.0554                                                                             8.4036                                                                             8.036 -6.4008                                                                            6.4367                                       __________________________________________________________________________

In this table, data are shown separately for the near axis systemhandling the lens area located near the optical axis and for thenonspherical system handling the peripheral portion.

As evident from Table 1, the screen which is not illustrated is ∞ inradius of curvature. (That is to say, the screen is a flat phase.) Theeffective radius (clap radius) assured in the optical performance is 600mm. The distance on the optical axis from the screen face to the face Slof the first lens 13 (denoted by the term "space between faces") is701.0 mm. The refractive index (of the air) between those faces is 1.0.

It is also shown that the face S1 of the first lens 13 has a radius ofcurvature of 92.9391 mm. The clap radius (effective radius in the viewof optical performance) is 37.0 mm. The space on the optical axisbetween the lens faces Sl and S2 (represented as "space between faces")is 7.0487 mm. The refractive index between those faces is shown to be1.4936.

In the same way, the fluorescent face 18 (S10) of the glass bulb 17 is-119.63 mm in radius of curvature and 60.0 mm in clap radius. The glassbulb 17 has thickness of 13.8 mm on the optical axis and refractiveindex of 1.5401.

Nonspherical coefficients are shown for faces S1 and S2 of the firstlens 13, faces S5 and S6 of the third lens 15, and the fluorescent face18 (S10).

The nonspherical coefficients are coefficients obtained when the faceshape is represented as ##EQU2## where Z represents the height of thelens face (as a function of r) when the direction of the optical axis istaken as the Z axis and the radial direction of the lens is taken as ther axis as shown in FIG. 2, and r and R_(D) represent the distance in theradial direction and the radius of curvature, respectively. Itcoefficients CC, DD, AE, AF and AG are given, therefore, the height,i.e., the shape of the lens face is defined by the above expression.

The contents of the data shown in Table 1 are understood by the abovedescription.

Since the second lens 14 and the fourth lens 16 of FIG. 1 are sphericallenses, all of their nonspherical coefficients are zero.

Tables 2 to 4 show concrete examples of other lens data.

Interpretation of these tables is the same as that of Table 1 and henceis not repetitively described.

                                      TABLE 2                                     __________________________________________________________________________                              Space                                               Paraxial      Radius of                                                                            Clap Between                                                                             Refractive                                    system        Curvature                                                                            Radius                                                                             Faces Index                                         __________________________________________________________________________    Screen     ∞                                                                          600                                                                                       701.0 1.0                                           First Lens S.sub.1                                                                          70.4536                                                                              37.0                                                                               9.7447                                                                              1.4936                                                   S.sub.2                                                                          98.1806                                                                              35.5                                                                               19.754                                                                              1.0                                           Second Lens                                                                              S.sub.3                                                                          75.5626                                                                              34.0                                                                               17.3  1.62286                                                  S.sub.4                                                                          -119.979                                                                             32.0                                                                               23.1726                                                                             1.0                                           Third Lens S.sub.5                                                                          586.226                                                                              34.0                                                                               5.012 1.4936                                                   S.sub.6                                                                          -139.059                                                                             34.0                                                                               23.0  1.0                                           Fourth Lens                                                                              S.sub.7                                                                          -40.676                                                                              34.0                                                                               3.0   1.5401                                                   S.sub.8        3.0   1.5401                                        Cooling Liquid            4.2   1.41                                                        ∞                                                                              60.0                                                     Glass                12.4 1.5401                                              Fluorescent Face S.sub.10                                                                   -224.52                                                                              60.0                                                     __________________________________________________________________________    Aspherical                                                                    coeffi-                                                                       cients     CC    AD   AE    AF   AG                                           __________________________________________________________________________                     × 10.sup.-7                                                                  × 10.sup.-9                                                                   × 10.sup.-13                                                                 × 10.sup.- 16                          First Lens                                                                             S.sub.1                                                                         -4.0025                                                                             -5.6627                                                                            -1.3450                                                                             -2.9026                                                                            3.3385                                                        × 10.sup.-7                                                                  × 10.sup.-10                                                                  × 10.sup.-13                                                                 × 10.sup.-16                                    S.sub.2                                                                         -3.3752                                                                             -4.4798                                                                            -9.8121                                                                             -1.5909                                                                            4.0946                                                        × 10.sup.-6                                                                  × 10.sup.-9                                                                   × 10.sup.-12                                                                 × 10.sup.-16                           Third Lens                                                                             S.sub.5                                                                         -3379.5                                                                             1.3346                                                                             -1.9910                                                                             1.7181                                                                             -6.4771                                                       × 10.sup.-7                                                                  × 10.sup.-10                                                                       × 10.sup.-17                                    S.sub.6                                                                         -31.2209                                                                            2.0209                                                                             7.2067                                                                              -2.43 ×  10.sup.-14                                                          6.2240                                       Fluorescent Face × 10.sup.-7                                                                  × 10.sup.-12                                                                  × 10.sup.-14                                                                 × 10.sup.-20                                    S.sub.10                                                                        -14,689                                                                             3.9240                                                                             -8.312                                                                              -1.601                                                                             2.4631                                       __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                              Space                                               Paraxial      Radius of                                                                            Clap Between                                                                             Refractive                                    system        Curvature                                                                            Radius                                                                             Faces Index                                         __________________________________________________________________________    Screen        ∞                                                                              600                                                                                701.0 1.0                                           First Lens S.sub.1                                                                          69.0430                                                                              37.0                                                                               9.0   1.4936                                                   S.sub.2                                                                          120.731                                                                              35.5                                                                               21.896                                                                              1.0                                           Second Lens                                                                              S.sub.3                                                                          73.7529                                                                              34.0                                                                               17.3  1.62286                                                  S.sub.4                                                                          -111.438                                                                             32.0                                                                               22.758                                                                              1.0                                           Third Lens S.sub.5                                                                          -6456.544                                                                            34.0                                                                               4.012 1.4936                                                   S.sub.6                                                                          -149.016                                                                             34.0                                                                               19.0  1.0                                           Fourth Lens                                                                              S.sub.7                                                                          -38.0291                                                                             33.5                                                                S.sub.8        3.0   1.5401                                        Cooling Liquid                                                                              ∞                                                                              60.0 4.0   1.41                                          Glass                     11.28 1.5401                                        Fluorescent Face S.sub.10                                                                   -261.57                                                                              60.0                                                     __________________________________________________________________________    Aspherical                                                                    coeffi-                                                                       cients     CC    AD   AE    AF   AG                                           __________________________________________________________________________                     × 10.sup.-7                                                                  × 10.sup.-9                                                                   × 10.sup.-13                                                                 × 10.sup.-16                           First Lens                                                                             S.sub.1                                                                         -3.9269                                                                             -5.3664                                                                            -1.3606                                                                             -4.1492                                                                            4.0028                                                        × 10.sup.-7                                                                  × 10.sup.-10                                                                  × 10.sup.-14                                                                 × 10.sup.-16                                    S.sub.2                                                                         -3.5202                                                                             -5.7934                                                                            -9.7981                                                                             -3.1520                                                                            3.5722                                                        × 10.sup.-7                                                                  × 10.sup.-9                                                                   × 10.sup.-12                                                                 × 10.sup.-15                           Third Lens                                                                             S.sub.5                                                                         -9548.79                                                                            7.0702                                                                             -1.4524                                                                             2.6650                                                                             -1.5654                                                       × 10.sup.-7                                                                  × 10.sup.-9                                                                   × 10.sup.-12                                                                 × 10.sup.-16                                    S.sub.6                                                                         -11.365                                                                             7.5329                                                                             2.5017                                                                              -1.2457                                                                            3.0134                                       Fluorescent Face × 10.sup.-8                                                                  × 10.sup.-11                                                                  × 10.sup.-14                                                                 × 10.sup.-18                                    S.sub.10                                                                        -54.824                                                                             5.5130                                                                             3.1435                                                                              1.6143                                                                             -8.0448                                      __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________                 Radius of    Space Between                                                                         Refractive                                               Curvature                                                                           Clap Radius                                                                          Faces   Index                                       __________________________________________________________________________    Paraxial                                                                           Screen  ∞                                                                             600                                                                                  701.0   1.0                                         System                                                                             First S.sub.1                                                                         84.9930                                                                             37.0                                                                                 4.981   1.4936                                           Lens  S.sub.2                                                                         119.696                                                                             35.5                                                                                 21.341  1.0                                              Second                                                                              S.sub.3                                                                         85.3919                                                                             34.0                                                                                 17.3    1.62286                                          Lens  S.sub.4                                                                         -102.165                                                                            32.0                                                                                 29.5926 1.0                                              Third S.sub.5                                                                         509.622                                                                             34.0                                                                                 7.012   1.4936                                           Lens  S.sub.6                                                                         -109.419                                                                            34.0                                                                                 20.0    1.0                                              Fourth                                                                              S.sub.7                                                                         -40.600                                                                             35.0                                                                                 3.0     1.5401                                           Lens  S.sub.8                                                                         ∞                                                                             60.0                                                            Cooling              3.0     1.41                                             Liquid                                                                              ∞                                                                         60.0                                                                  Glass                14.0    1.5401                                           Fluorescent                                                                           -300.0                                                                              60.0                                                            Face S.sub.10                                                            __________________________________________________________________________                 CC   AD    AE    AF   AG                                         __________________________________________________________________________    Aspherical                                                                          First                                                                             S.sub.1                                                                          -3.7010                                                                            × 10.sup.- 6                                                                  × 10.sup.-9                                                                   × 10.sup.-12                                                                 × 10.sup.-16                         coeffi-           -1.6725                                                                             -3.4272                                                                             2.6518                                                                             -5.4293                                    cients                                                                              Lens                                                                              S.sub.2                                                                          2.1420                                                                             × 10.sup.-6                                                                   × 10.sup.-9                                                                   × 10.sup.-12                                                                 × 10.sup.-16                                           -1.5377                                                                             -2.5504                                                                             2.4882                                                                             -4.6114                                          Third                                                                             S.sub.5                                                                          -5149.72                                                                           × 10.sup.-6                                                                   × 10.sup.-10                                                                  × 10.sup.-13                                                                 × 10.sup.-16                                           1.0428                                                                              -7.1879                                                                             -2.0677                                                                            6.3138                                           Lens                                                                              S.sub.6                                                                          -0.56697                                                                           × 10.sup.-7                                                                   × 10.sup.-9                                                                   × 10.sup.-12                                                                 × 10.sup.-15                                           6.7685                                                                              1.3022                                                                              -1.6193                                                                            1.1021                                     __________________________________________________________________________

In the optical system according to the present invention, it becomespossible to largely shorten the projection distance and improve thefocus performance as compared with the prior art by using lenses and theBraun tube configured as heretofore described.

Features of the present invention will now be described.

All of the data shown in Tables 1 to 4 correspond to embodiments withmagnification 9.5 to 10. That is to say, the raster size on thefluorescent face of the Braun tube is chosen to be 4.5 inch in diagonalline length, and it is projected onto the screen to be enlarged to 45inch.

In reality, however, distortion of 5 to 8% exists as described later.Therefore, the magnification on the near axis assumes a value somewhatless than 10, i.e., 9.2 to 9.5.

The distance from the front end of the lens to the screen isapproximately 700 mm, which is approximately 55% as compared with 1,200to 1,300 mm of the conventional projection television receiver.

Further, these lenses have F number 1.0 and hence have very highluminance, assuming the top rank in the projection lenses available now.

In general, such as increase in picture angle increases the brightnessor aberration of the peripheral portion. The present invention hassolved these problems by using a method comprising the steps ofattempting optimum design of lenses and providing the fluorescent faceof the Braun tube with curvature.

The peripheral light quantity I_(c) is generally given by

    I.sub.c ∝V·cos.sup.4 θ

where θ is the picture angle and V is vignetting. The value of cos⁴ θfor picture angle of 40° is nearly half that for picture angle of 25°.

In order to compensate the resulting deterioration of the lightquantity, vignetting of the present invention is increased to twice ascompared with the conventional case where the picture angle is 25°. Thusthe synthetic peripheral light quantity is maintained at a value notless than that of the prior art. The focus characteristics estimated byusing MTF are shown in FIGS. 3 to 6.

That is to say, FIGS. 3 to 6 are characteristic diagrams correspondingto Tables 1 to 4, respectively.

The values of 1 pl/mm (pair lines/mm), 2 pl/mm and 3 pl/mm indicatefrequencies of black and white stripe signals on the Braun tube andrepresent 137 TV lines, 274 TV lines and 411 TV lines, respectively.

For TV pictures, the view point is in many cases located near thecenter. Accordingly, the picture quality of the central portion becomesmore important than that of the peripheral portion. As evident fromFIGS. 3 to 6, a major feature of the optical system according to thepresent invention is that little deterioration is found in the arearanging from the central portion to the picture angle of 50%. Further,the MTF characteristics of the peripheral portion are also much betterthan those of lenses for domestic projection television receivers in usenow.

FIG. 7 shows the distortion obtained for the embodiment shown in Table1.

Since mearly the same tendency is found in other embodiments, theresulting distortion will now be described by taking this embodiment asa representative one.

In case of television sets, compensation by a circuit and compensationby the deflection yoke are applied to the distortion. The distortion, ifany, appears on the picture as a raster distortion and linearitydeterioration. Especially, the raster distortion poses a problem.

FIGS. 8 and 9 show the raster distortion resulting from the distortionshown in FIG. 7. FIG. 8 shows the raster distortion in the verticaldirection. FIG. 9 shows the raster distortion in the horizontaldirection.

In FIGS. 8 and 9, (A) represents the characteristics and (B) showsdimensions in the picture plane.

As described before, most projection television sets now in use havecircuits for compensating raster distortion. However, this compensationcircuit provides linear compensation proportioned to the picture angle.It is difficult to compensate nonlinear patterns. An attempt tocompensate a nonlinear pattern results in an increased cost.

In FIGS. 8 and 9, the raster distortion after circuit compensation isshown as residual distortion. It is seen in these figures that theresidual distortion is 1% or less in the same way as the conventionallens, posing no problem.

Table 5 summarizes quantities listed below for the embodiments of Tables1 to 5.

                  TABLE 5                                                         ______________________________________                                        Embodiments  P.sub.1 /P.sub.0                                                                        P.sub.2 /P.sub.0                                                                        P.sub.3 /P.sub.0                                                                     t/f                                   ______________________________________                                        1 (Table 1)  0.10      0.88      0.40   0.26                                  2 (Table 2)  0.14      0.88      0.30   0.34                                  3 (Table 3)  0.22      0.92      0.22   0.28                                  4 (Table 4)  0.12      0.88      0.37   0.29                                  ______________________________________                                    

It is seen in Table 5 that relations

    ______________________________________                                                  0.05 < P.sub.1 /P.sub.0 < 0.25                                                0.85 < P.sub.2 /P.sub.0 < 0.95                                                0.2 < P.sub.3 /P.sub.0 < 0.45                                                 0.25 < t/f < 0.35                                                   ______________________________________                                    

hold true.

In any of embodiments shown in Tables 1 to 4, the second lens 14 iscomposed of a glass lens having a spherical shape. If a plastic lens isused, the image face is shifted and the focus is deteriorated because ofa change in refractive index caused by a change in temperature or achange in shape caused by thermal expansion. In accordance with thepresent invention, the glass lens (the second lens 14 in the concreteform) is provided with large power as a countermeasure. The fourth lens16 is also composed of glass lens having a spherical shape.

This aims at simplify the coupling structure between the lens and theBraun tube face to improve the reliability and cut the cost.

If this lens is made of plastic, the reliability is lowered as describedbelow. For example, the plastic lens may be attacked by the coolingliquid and its surface may be degenerated. Or the plastic lens may havea high temperature and may be subjected to thermal deformation, becausethe plastic lens is located near the fluorescent face having highluminance and a high temperature. Occurrence of these problems resultsin the lowered reliability.

FIG. 10 shows another embodiment of the present invention.

A fluorescent face 105 of Braun tube, a screen 106, a first lens 107, asecond lens 108, a third lens 109, a fourth lens 110, cooling liquid111, and a glass bulb 112 are illustrated in FIG. 10.

The optical system shown in FIG. 10 is so configured as to exhibit thebest performance when the raster having a length of 4.5 inch on thefluorescent face 105 of Braun tube is enlarged to a raster having alength of 45 inch on the screen 106, i.e., when the magnification is 10.The distance from the first lens 107 to the screen 106, which is theso-called projection distance, is 700 mm. The picture angle isapproximately 40 degrees. It is understood that an ultrawide angle lensis formed as compared with the projection distance of 1,200 mm and thepicture angle of approximately 25 degrees of the conventional lenssystem.

The first lens 107 has a nonspherical shape to eliminate the sphericalaberration based upon the aperture. The second lens 108 is composed of aglass lens in order to decrease the focus drift caused by a temperaturechange and is provided with power as much as possible. The third lens109 has a nonspherical shape in order to eliminate the high order comaand astigmatism and is provided with power as little as possible. Thefourth lens 110 is a lens for compensating the curve of the image face.Since the fourth lens 110 is subjected to heat generated by the Brauntube as described before, it is made of glass to withstand the heat. Inorder to cut the cost as much as possible, the fourth lens 110 is formedas a concave meniscus lens.

The fluorescent face 105 of the Braun tube is provided with curvature inorder to compensate the curvature of field. The fluorescent face 105 hasa nonspherical shape especially for compensating the high order curve ofthe image face. In general, the fluorescent face side of the Braun tubeundergoes only the press working and does not undergo the subsequentworking. In spite of the nonspherical shape, therefore, themanufacturing method is not changed. In the central portion, the lenssystem (optical system) according to the present invention has nearlyzero spherical aberration and produces only the color aberration. Ascompared with the lens system described in the aforementioned U.S. Pat.No. 4,300,817, the color aberration is reduced by the ratio of 700/1,200and the center focus is improved by approximately 40%.

The plastic lens used in the present lens system has very little power.That is to say, the lens is thin and the thickness difference betweenthe center and the periphery is small. Thus the lens can be easilyformed. The concrete lens data which can be assumed by the present lenssystem are shown in Tables 6 to 9. In every table, the F number is 1.0and very high luminance is achieved. The picture angle is 40 degrees.

                                      TABLE 6                                     __________________________________________________________________________                  Radius of                                                                     Curvature   Space Between                                                                         Refractive                                                (RD) Clap Radius                                                                          Faces   Index                                       __________________________________________________________________________    Paraxial                                                                           Screen   ∞                                                                            600                                                        system                    701.0   1.0                                              First S.sub.1                                                                          89.5209                                                                            40.0                                                                                 7.049   1.4936                                           Lens  S.sub.2                                                                          144.387                                                                            36.5                                                                                 21.124  1.0                                              Second                                                                              S.sub.3                                                                          75.429                                                                             34.0                                                                                 18.3    1.59096                                          Lens  S.sub.4                                                                          -114.739                                                                           31.5                                                                                 21.392  1.0                                              Third S.sub.5                                                                          795.357                                                                            34.0                                                                                 6.012   1.4936                                           Lens  s.sub.6                                                                          -155.305                                                                           34.0                                                                                 25.6    1.0                                              Fourth                                                                              S.sub.7                                                                          -38.854                                                                            34.5                                                            Lens  S.sub.8                                                                          -100.0                                                                             65.0   3.0     1.51827                                          Cooling              1.939   1.41                                             Liquid   ∞                                                                            65.0                                                            Glass                13.3    1.5401                                           Fluorescent                                                                         171.05                                                                           65.0                                                                 Face P.sub.1                                                             __________________________________________________________________________                  CC   AD   AE   AF    AG                                         __________________________________________________________________________    Aspherical                                                                          First S.sub.1                                                                         -3.5246                                                                            × 10.sup.- 6                                                                 × 10.sup.-9                                                                  × 10.sup.-12                                                                  × 10.sup.-16                         coeffi-            -1.4882                                                                            -3.3451                                                                            2.5818                                                                              -5.5038                                    cients                                                                              Lens  S.sub.2                                                                         2.6171                                                                             × 10.sup.-6                                                                  × 10.sup.-9                                                                  × 10.sup.-12                                                                  × 10.sup.-16                                            -1.4875                                                                            -2.6763                                                                            2.6856                                                                              -6.0086                                          Third S.sub.5                                                                         -254.18                                                                            × 10.sup.-7                                                                  × 10.sup.-10                                                                 × 10.sup.-13                                                                  × 10.sup.-16                                            6.5455                                                                             -3.3198                                                                            8.2570                                                                              -3.5901                                          Lens  S.sub.6                                                                         -8.0572                                                                            × 10.sup.-6                                                                  × 10.sup.-10                                                       1.0179                                                                             7.8019                                                                             0     0                                                Fluorescent                                                                         P.sub.1                                                                         -1.7 × 10.sup.-7                                                                  × 10.sup.-11                                                                 × 10.sup.-14                                                                  × 10.sup.-18                                            5.295                                                                              5.063                                                                              -4.033                                                                              4.0551                                     __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________                  Radius of                                                                     Curvature   Space Between                                                                         Refractive                                                (RD) Clap Radius                                                                          Faces   Index                                       __________________________________________________________________________    Paraxial                                                                           Screen   ∞                                                                            600                                                                                  701.0   1.0                                              First S.sub.1                                                                          93.390                                                                             40.0                                                                                 7.049   1.4936                                           Lens  S.sub.2                                                                          133.892                                                                            37.5                                                                                 18.843  1.0                                              Second                                                                              S.sub.3                                                                          77.574                                                                             34.0                                                                                 17.3    1.59096                                          Lens  S.sub.4                                                                          -106.273                                                                           32.0                                                                                 24.974  1.0                                              Third S.sub.5                                                                          249.038                                                                            34.0                                                                                 8.012   1.4936                                           Lens  S.sub.6                                                                          -208.647                                                                           34.0                                                                                 22.6    1.0                                              Fourth                                                                              S.sub.7                                                                          -38.823                                                                            35.0                                                            Lens  S.sub.8                                                                          -100.0                                                                             65.0   3.0     1.51827                                          Cooling              2.0     1.41                                             Liquid   ∞                                                                            65.0                                                            Glass                13.8    1.5401                                           Fluorescent                                                                            -155.52                                                                            65.0                                                            Face P.sub.1                                                             __________________________________________________________________________                  CC   AD   AE   AF    AG                                         __________________________________________________________________________    Aspherical                                                                          First S.sub.1                                                                         -4.8137                                                                            × 10.sup.- 6                                                                 × 10.sup.-9                                                                  × 10.sup.-12                                                                  × 10.sup.-16                         coeffi-            -1.5886                                                                            -3.3460                                                                            2.5926                                                                              -5.6081                                    cients                                                                              Lens  S.sub.2                                                                         2.3921                                                                             × 10.sup.-6                                                                  × 10.sup.-9                                                                  × 10.sup.-12                                                                  × 10.sup.-16                                            -1.5798                                                                            -2.5927                                                                            2.6404                                                                              -5.8014                                          Third S.sub.5                                                                         -68.5873                                                                           × 10.sup.-7                                                                  × 10.sup.-10                                                                 × 10.sup.-13                                                                  × 10.sup.-17                                            6.9339                                                                             -5.6883                                                                            -1.6146                                                                             -5.1634                                          Lens  S.sub.6                                                                         -143.9                                                                             × 10.sup.-7                                                                  × 10.sup.-9                                                                  × 10.sup.-12                                                                  × 10.sup.-16                                            -8.3110                                                                            1.6224                                                                             -1.2352                                                                             1.9594                                           Fluores-                                                                            P.sub.1                                                                         -3.0554                                                                            × 10.sup.-7                                                                  × 10.sup.-11                                                                 × 10.sup.-14                                                                  × 10.sup.-18                               cent Face    5.8835                                                                             5.625                                                                              -4.4810                                                                             4.5061                                     __________________________________________________________________________

                                      TABLE 8                                     __________________________________________________________________________                  Radius of                                                                     Curvature   Space Between                                                                         Refractive                                                (RD) Clap Radius                                                                          Faces   Index                                       __________________________________________________________________________    Paraxial                                                                           Screen                                                                              ∞                                                                          600                                                             system                    701.0   1.0                                              First S.sub.1                                                                          91.375                                                                             40.0                                                                                 7.049   1.4936                                           Lens  S.sub.2                                                                          136.144                                                                            37.5                                                                                 18.453  1.0                                              Second                                                                              S.sub.3                                                                          79.961                                                                             34.0                                                                                 17.3    1.59096                                          Lens  S.sub.4                                                                          -104.859                                                                           32.0                                                                                 24.496  1.0                                              Third S.sub.5                                                                          246.480                                                                            34.0                                                                                 8.012   1.4936                                           Lens  S.sub.6                                                                          -182.659                                                                           34.0                                                                                 22.6    1.0                                              Fourth                                                                              S.sub.7                                                                          -40.024                                                                            35.0                                                            Lens  S.sub.8                                                                          -100.0                                                                             65.0   3.0     1.62325                                          Cooling              2.0     1.41                                             Liquid   ∞                                                                            65.0                                                            Glass                13.8    1.5401                                           Fluorescent                                                                            -155.52                                                                            65.0                                                            Face P.sub.1                                                             __________________________________________________________________________                  CC   AD   AE   AF    AG                                         __________________________________________________________________________    Aspherical                                                                          First S.sub.1                                                                         -4.6508                                                                            × 10.sup.- 6                                                                 × 10.sup.-9                                                                  × 10.sup.-12                                                                  × 10.sup.-16                         coeffi-            -1.6176                                                                            -3.3670                                                                            2.5887                                                                              -5.5889                                    cients                                                                              Lens  S.sub.2                                                                         2.2709                                                                             × 10.sup.-6                                                                  × 10.sup.-9                                                                  × 10.sup.-12                                                                  × 10.sup.-16                                            -1.6139                                                                            -2.5776                                                                            2.6319                                                                              -5.8520                                          Third S.sub.5                                                                         -156.527                                                                           × 10.sup.-6                                                                  × 10.sup.-10                                                                 × 10.sup.-13                                                                  × 10.sup.-17                                            1.3274                                                                             -2.1805                                                                            -2.2953                                                                             3.2074                                           Lens  S.sub.6                                                                         -34.167                                                                            × 10.sup.-8                                                                  × 10.sup.-9                                                                  × 10.sup.-13                                                                  × 10.sup.-16                                            -7.2389                                                                            1.5732                                                                             -9.1082                                                                             1.2742                                           Fluores-                                                                            P.sub.1                                                                         -3.0554                                                                            × 10.sup.-7                                                                  × 10.sup.-11                                                                 × 10.sup.-14                                                                  × 10.sup.-18                               cent Face    5.8835                                                                             5.6250                                                                             -4.4810                                                                             6.5061                                     __________________________________________________________________________

                                      TABLE 9                                     __________________________________________________________________________                  Radius of                                                                     Curvature   Space Between                                                                         Refractive                                                (RD) Clap Radius                                                                          Faces   Index                                       __________________________________________________________________________    Paraxial                                                                           Screen                                                                              ∞                                                                          600                                                             system                    701.0   1.0                                              First S.sub.1                                                                          70.4536                                                                            37.0                                                                                 9.745   1.4936                                           Lens  S.sub.2                                                                          100.427                                                                            35.5                                                                                 19.175  1.0                                              Second                                                                              S.sub.3                                                                          74.577                                                                             34.0                                                                                 17.3    1.5909                                           Lens  S.sub.4                                                                          -113.749                                                                           32.0                                                                                 23.66   1.0                                              Third S.sub.5                                                                          633.047                                                                            34.0                                                                                 5.012   1.4936                                           Lens  S.sub.6                                                                          -155.560                                                                           34.0                                                                                 23.0    1.0                                              Fourth                                                                              S.sub.7                                                                          -38.221                                                                            34.5                                                            Lens  S.sub.8                                                                          -100.0                                                                             65.0   3.0     1.51827                                          Cooling              4.2     1.41                                             Liquid   ∞                                                                            65.0                                                            Glass                12.4    1.5401                                           Fluorescent                                                                            -224.52                                                                            65.0                                                            Face P.sub.1                                                             __________________________________________________________________________                  CC   AD   AE   AF    AG                                         __________________________________________________________________________    Aspherical                                                                          First S.sub.1                                                                         -3.7652                                                                            × 10.sup.- 7                                                                 × 10.sup.-9                                                                  × 10.sup.-13                                                                  × 10.sup.-16                         coeffi-            -5.2558                                                                            -1.3265                                                                            -2.8979                                                                             3.1657                                     cients                                                                              Lens  S.sub.2                                                                         -2.7480                                                                            × 10.sup.-7                                                                  × 10.sup.-10                                                                 × 10.sup.-13                                                                  × 10.sup.-16                                            -4.5675                                                                            -9.1198                                                                            -1.3062                                                                             3.4793                                           Third S.sub.5                                                                         -4555.890                                                                          × 10.sup.-6                                                                  × 10.sup.-9                                                                  × 10.sup.-12                                                                  × 10.sup.-16                                            1.4793                                                                             -1.8907                                                                            1.8922                                                                              -5.3263                                          Lens  S.sub.6                                                                         -37.727                                                                            × 10.sup.-7                                                                  × 10.sup.-10                                                                 × 10.sup.-14                                                                  × 10.sup.-16                                            3.6028                                                                             8.2319                                                                             1.7245                                                                              1.8408                                           Fluores-                                                                            P.sub.1                                                                         -14.689                                                                            × 10.sup.-7                                                                  × 10.sup.-12                                                                 × 10.sup.-14                                                                  × 10.sup.-20                               cent Face    3.924                                                                              -8.312                                                                             -1.6010                                                                             2.4631                                     __________________________________________________________________________

The screen 106 is ∞ in radius of curvature. (That is to say, the screen106 is a flat plane.) The effective radius (clap radius) assured in theoptical performance is 600 mm. The distance on the optical axis from thescreen face to the face S1 of the first lens 107 (denoted by the term"space between faces") is 701.0 mm. The refractive index (of the air)between those faces is 1.0.

The face S1 of the first lens 107 has a radius of curvature of 89.5209mm. The clap radius (effective radius in the view of opticalperformance) is 40.0 mm. The space on the optical axis between the lensfaces S1 and S2 (represented as "space between faces") is 7.049 mm. Therefractive index between those faces is 1.4936.

It is shown that the fluorescent face 105 (P1) of the glass bulb 112 is-171.05 mm in radius of curvature and 65.0 mm in clap radius. The glassbulb 112 has thickness of 13.3 mm on the optical axis and refractiveindex of 1.5401.

Nonspherical coefficients are shown for faces S1 and S2 of the firstlens 107, faces S5 and S6 of the third lens 109, and the fluorescentface 105 (P1).

As described before, the aspherical coefficients are coefficientsobtained when the face shape is represented as ##EQU3## where Zrepresents the height of the lens face (as a function of r) when thedirection of the optical axis is taken as the Z axis and the radialdirection of the lens is taken as the r axis as shown in FIG. 2, and rand R_(D) represent the distance in the radial direction and the radiusof curvature, respectively. If coefficients CC, AD, AE, AF and AG aregiven, therefore, the height, i.e., the shape of the lens face isdefined by the above expression.

Since the second lens 108 and the fourth lens 110 of FIG. 10 arespherical lenses, all of their non-spherical coefficients are zero.

In the optical system according to the present invention, it becomespossible to largely shorten the projection distance and improve thefocus performance as compared with the prior art by using lenses and theBraun tube configured as heretofore described. FIGS. 11 to 14 show thefocus characteristics estimated by using MTF (modulation transferfunction) when an image of 4.5 inch on the fluorescent face is soprojected onto the screen as to be enlarged to an image of 45 inch byusing the optical system according to the present invention heretoforedescribed. Characteristics shown in FIGS. 11 to 14 correspond to Tables6 to 9, respectively.

Data are plotted for 140 TV lines and 280 TV lines taken as thefrequency of black and white stipe signals on the Braun tube.

When averaged for all picture angles, the MTF characteristics of 280 TVlines are improved by 50% or more as compared with those described inU.S. Pat. No. 4,526,442.

FIG. 15 is a distortion characteristic diagram for the embodiment shownin Table 6.

For other embodiments as well, similar values are obtained. FIG. 15shows a somewhat large value 7.3% of distortion. However, the distortioncharacteristics follow a monotonic function. By generating barreldistortion on the Braun tube, it is possible to eliminate the distortionand generate a raster on the screen without distortion.

The barrel distortion can be generated by letting flow a compensationcurrent through the deflection yoke or letting flow a compensationcurrent through the convergence yoke disposed at the rear part of thedeflection yoke.

In the optical system according to the present invention, the projectiondistance is reduced as compared with the conventional optical system andat the same time the overall dimensions are also significantly reduced.Both the length and aperture of the lens system are shortened by 30 to50% as compared with those described in U.S. Pat. No. 4,526,442.

As a result, it becomes possible to realize an inexpensive lens systemand a compact projection television set.

Assuming that powers of the whole system, the first lens, the secondlens, the third lens and the fourth lens are respectively P₀, P₁, P₂, P₃and P₄ in the embodiments shown in Tables 6 to 9, relations shown inTable 10 hold true.

                  TABLE 10                                                        ______________________________________                                        Embodiments  P.sub.1 /P.sub.0                                                                       P.sub.2 /P.sub.0                                                                       P.sub.3 /P.sub.0                                                                     P.sub.4 /P.sub.0                        ______________________________________                                        1 (Table 6)  0.15     0.88     0.27   0.86                                    2 (Table 7)  0.13     0.88     0.30   0.84                                    3 (Table 8)  0.13     0.88     0.32   0.93                                    4 (Table 9)  0.16     0.89     0.28   0.87                                    ______________________________________                                    

That is to say, it is understood that the following relations hold true:

    ______________________________________                                                  0.1 < P.sub.1 /P.sub.0 < 0.19                                                 0.86 < P.sub.2 P.sub.0  < 0.9                                                 0.25 < P.sub.3 /P.sub.0 < 0.33                                                0.82 < P.sub.4 /P.sub.0 < 0.95                                      ______________________________________                                    

The nonspherical shape of the fluorescent face can be described byreferring to FIG. 16 as follows.

FIG. 16 is a diagram for illustrating the nonspherical shape of thefluorescent face. As shown in FIG. 16, the radius of curvature on thecenter axis is represented by R. Assume that a first point is located onthe fluorescent face and is 2.25 inch or 57 mm from the center in thedirection perpendicular to the central axis. And assume a second pointis located on the intersection of the central axis and the fluorescentface. The deviation in the central axis direction between the firstpoint and the second point is represented by ΔZ. If the fluorescent faceis spherical, the relation

    ΔZ=(57.sup.2)/(2R)

holds true. In this case, however, the fluorescent face is not sphericalbut nonspherical. Therefore, the above relation does not hold true andslight deviation is caused. Letting (57²)/(2R)=ΔZ₁, Table 11 comparesthe actual values of ΔZ with ΔZ₁ to show the shift for respectiveembodiments shown in Tables 6 to 9.

                  TABLE 11                                                        ______________________________________                                         Embodiments                                                                             R                                                                                      ##STR1##     ΔZ                                                                            ΔZ/ΔZ.sub.1                ______________________________________                                        Table 6   ⊖171.05                                                                        9.50         ⊖5.02                                                                       0.53                                    Table 7   ⊖155.5                                                                         10.4         ⊖5.03                                                                       0.48                                    Table 8   ⊖155.5                                                                         10.4         ⊖5.03                                                                       0.48                                    Table 9   ⊖224.5                                                                         7.24         ⊖4.02                                                                       0.55                                    ______________________________________                                    

It is evident from the data shown in Table 11 that the followingrelations hold true:

    0.4<ΔZ/ΔZ.sub.1 <0.7

    -300<R<-100

We claim:
 1. An optical system of a projection television receiver forprojecting pictures from a picture tube onto a screen, comprising a tubeface of the picture tube curved in such a direction as to compensate thecurve of the image face of the projected picture on the screen, andfurther comprising lenses arranged from the screen side to the tube faceof the receiver tube in the order of:(A) a first lens having power(reciprocal of focal distance) P₁ related to power P₀ of the wholeoptical system as
 0. 05<P₁ /P₀ <0.25(B) a second glass lens of biconvexform having power P₂ related to said P₀ as

    0.85<P.sub.2 /P.sub.0 <0.95

(C) a third lens having power P₃ related to said P₀ as

    0.2<P.sub.3 /P.sub.0 <0.45; and

(D) a fourth lens comprising a concave lens disposed to compensate thecurve of the image face of the projected picture on the screen; whereinsaid third lens is spaced from said second glass lens by a distancelarger than a thickness of said third lens.
 2. An optical system of aprojection television receiver according to claim 1, wherein the ratioof the distance t between the third lens and the fourth lens to thefocal distance f of the whole optical system lies in the range

    0.25<t/f<0.35.


3. An optical system of a projection television receiver according toclaim 1, wherein at least one face is nonspherical in each of the firstlens and the third lens.
 4. An optical system of a projection televisionreceiver according to claim 1, wherein the fourth lens comprisesspherical glass.
 5. An optical system of a projection televisionreceiver according to claim 1, wherein the space between the fourth lensand the tube face of the picture tube is filled with cooling liquid. 6.An optical system of a projection television receiver according to claim1, wherein the distortion causing picture distortion on the screen liesin the range of -5% to -9% and forms a monotonically increasing functionwith respect to the picture angle.
 7. An optical system for a projectionsystem for projecting a picture reproduced on an image display deviceonto a screen, comprising from the screen side:a first lens elementhaving a weak power and aspherical surfaces at both sides; a secondglass lens element of biconvex form providing a majority of positivepower of the overall optical system; a third lens element of relativelyweak positive peak having aspherical surfaces at both sides; and afourth lens element of negative power having a surface which is stronglyconcave to the screen side; wherein
 0. 85<P₂ /P₀ <0.95 where P2 is thepower of said second glass lens element and P0 is the power of the wholeoptical system, and said third lens element is spaced from said secondglass lens element by a distance larger than a thickness of said thirdlens element.
 8. An optical system according to claim 7, wherein

    0.05<P1/P0<0.25

where P1 is the power of said first lens element and P0 is the power ofthe whole optical system.
 9. An optical system according to claim 7,wherein

    0.2<P3/P0<0.45

where P3 is the power of said third lens element and P0 is the power ofthe whole optical system.
 10. A projection system comprising:an imagedisplay device for reproducing a picture to be projected; a screen; andan optical means disposed between said image display device and saidscreen for projecting said picture onto said screen, said optical meanscomprising from the screen side, a first lens element having a weakpower and aspherical surfaces at both sides, a second glass lens elementof biconvex form providing a majority of positive power of the overalloptical system, a third lens element of relatively weak positive powerhaving aspherical surfaces at both sides; and a fourth lens element ofnegative power having a surface which is strongly concave to the screenside, wherein
 0. 85<P₂ /P₀ <0.95 where P2 is the power of said secondglass lens element and P0 is the power of the whole optical system andsaid third lens element is spaced from said second lens element by adistance larger than a thickness of said third lens element.
 11. Anprojection system according to claim 10, wherein

    0.05<P1/P0<0.25

where P1 is the power of said first lens element and P0 is the power ofthe whole optical system.
 12. An projection system according to claim10, wherein

    0.2<P3/P0<0.45

where P3 is the power of said third lens element and P0 is the power ofthe whole optical system.
 13. An optical system for a projection systemfor projecting a picture reproduced on a curved phosphor surface of apicture tube onto a screen, said curved phosphor surface being concaveto the image end to contribute to correction of field curvaturedistortion in the projected picture which may be caused if the phosphorsurface is plano, comprising from the screen side;a first lens elementhaving a weak power and aspherical surface at both sides; a second glasslens element of biconvex form providing a majority of positive power ofthe overall optical system; a third lens element of relatively weakpositive power having aspherical surfaces at both sides; and a fourthlens element of negative power having a surface with is strongly concaveto the screen side; wherein
 0. 85<P₂ /P₀ <0.95 where P2 is the power ofsaid second glass lens element and P0 is the power of the whole opticalsystem, and said third lens element is spaced from said second glasslens element by a distance larger than a thickness of said third lenselement.
 14. An optical system according to claim 13, wherein

    0.05<P1/P0<0.25

where P1 is the power of said first lens element and P0 is the power ofthe whole optical system.
 15. An optical system according to claim 13,wherein

    0.2<P3/P0<0.45

where P3 is the power of said third lens element and P0 is the power ofthe whole optical system.
 16. An optical system according to claim 13,wherein said curved phosphor surface is aspherical.
 17. An opticalsystem according to claim 13, further comprising coupling meansconnected between said fourth lens element and said picture tube andhaving a cooling liquid provided therein.
 18. A projection systemcomprising:a screen; a picture tube having a curved phosphor surface onwhich a picture to be projected onto a screen is reproduced, said curvedphosphor surface being concave to the image end to contribute tocorrection of field curvature distortion in a projected picture whichmay be caused if the phosphor surface is plano; and an optical meansdisposed between said image display device and said screen forprojecting said picture onto said screen, said optical means comprisingfrom the screen side, a first lens element having a weak power andaspherical surfaces at both sides, a second glass lens element ofbiconvex form providing a majority of positive power of the overalloptical system, a third lens element of relatively weak positive powerhaving aspherical surfaces at both sides, and a fourth lens element ofnegative power having a surface which is strongly concave to the screenside; wherein

    0.85<P.sub.2 /P.sub.0 <0.95

where P2 is the power of said second glass lens element and P0 is thepower of the whole optical system, and said third lens element is spacedfrom said second glass lens element by a distance larger than athickness of said third lens element.
 19. An projection system accordingto claim 18, wherein

    0.05<P1/P0<0.25

where P1 is the power of said first lens element and P0 is the power ofthe whole optical system.
 20. A projection system according to claim 18,wherein

    0.2<P3/P0<0.45

where P3 is the power of said third lens element and P0 is the power ofthe whole optical system.
 21. A projection system according to claim 18,wherein said curved phosphor surface is aspherical.
 22. An opticalsystem according to claim 18, further comprising coupling meansconnected between said fourth lens element and said picture tube andhaving a cooling liquid provided therein.
 23. An optical system forprojecting an image, comprising:a cathode ray tube having a face glass,said face glass having a convex surface to which a plurality of phoshorelements are mounted, side surface of said face glass to which saidphosphor elements are mounted being aspheric; a first aspheric lens ofrelatively weak power; a second biconvex lens of glass for providing amajor portion of enlargement provided by the optical system; a thirdaspheric lens of relatively weak positive power; and a fourth lens ofrelatively large negative power with a concave face on the screen sidethereof; wherein

    0.85<P.sub.2 /P.sub.0 <0.95

where P2 is the power of said second lens and P0 is the power of thewhole optical system, and said third lens is spaced from said secondlens by a distance larger than a thickness of said third lens element.24. An optical system for a projection television, comprising:a firstlens element having a weak power and an aspherical surface; a secondglass lens element of biconvex form providing a majority of the positivepower of the overall optical system; a third lens element of relativelyweak power having an aspherical surface; and a fourth lens element ofnegative power having a surface which is strongly concave to the screenside; wherein

    0.85<P2/P0<0.95

where P2 is the power of said second lens element and P0 is the power ofthe whole optical system, and said third lens element is spaced fromsaid second lens element by a distance larger than a thickness of saidthird lens element.